Automatic bar feeders for mono-spindle and multi-spindle machine tools are generally known, comprising pushers which operate to insert and move each bar forwards by steps along a bar guide means axially aligned with a spindle of the machine tool. U.S. Pat. Nos. 3,612,298, 3,955,687, and 4,407,176 are some examples of devices for feeding elongated bars on mono-spindle and multi-spindle turning machines.
Nevertheless bar feeders having single or double pusher, of the known type, have considerable disadvantages which depend on their structure and on the functional organisation of the feeder itself. In particular the use of a single acting pushing member require feeding device of considerable length furtherly involving long time for returning the pushing member and to reintroduce a new bar in the spindle of the machine.
In order to reduce the longitudinal dimensions of the feeder, EP-A- 26370 suggests the use of a double pushing device in which first and second pushing members are provided on a same carriage slidable on a guide rod parallelly arranged to guide channel axially aligned to a guide tube. The first pushing member during forwards stroke of the carriage operate to partially introduce a bar into the guide tube, while entraining the second pushing member on the same bar, then the carriage with both pushing members is retracted to dispose their second pushing member in alignment with the bar to furtherly introduce the latter during a second active stroke of the entraining carriage to feed the bar into the spindle of a machine.
Although said feeding device allows to substantially reduce the length of the same feeder, nevertheless it require long time consuming for returning the carriage to the starting position to activate the second pushing member of the device; the time lost for returning the carriage with the second pushing member therefore reduces the productivity of the machine.
In order to reduce noise and to improve bar charging operations in mono-spindle machines, U.S. Pat. No. 5,066,185 suggest the use of a carriage axially aligned to the spindle of the machine, to act as first pushing member to partially forward bars in the same working machine; a second pushing member is provided to move the bars in the final portion of its forward stroke, nevertheless both the pushers and the means for guiding the bars are structured and driven to move a bar forwards by steps along the spindle, at each working cycle, while the machine tool is in operation. Consequently, when the first pusher has completed its forward stroke, after the bar has been partially machined, the machine tool has to be stopped, the first pusher has to be replaced by the second one with the machine at a standstill and subsequently actuated to move the remaining part of the bar forwards by steps once again after the machine has been again started to operate.
In bar feeders with double pusher of the known type, it is therefore necessary to stop the machine to replace the pushing members, with consequent waste of time and productivity; furthermore in machine tools operating at extremely high speeds, stopping may cause impact and damage to the spindle and to the working tools.
Therefore, the scope of the present invention is to provide an automatic bar feeding device for machine tools, having double pushing device of the kind mentioned above, suitable designed for avoiding the above mentioned disadvantages.
A further scope of present invention is to provide a double-pusher bar feeding device which substantially allows to reduce the loading time of the bars, and devisable for use in mono-spindle and multi-spindle working machines.
The above mentioned scopes are accomplished by means of an automatic bar feeder complying with the characteristic features of the main claim.